The present technology generally relates to the generation of digital images. More specifically, the technology relates to the use of forward projection to generate a computed tomography (CT) image under arbitrary tube spectra by a dual energy scan.
Dual energy imaging essentially involves taking multiple scans of the same target under the same conditions at two energy spectra. In a Dual Energy system, multiple scans are performed at the different energy levels (or energy spectra), and are used to identify different materials. For example, soft tissue and other lower density elements tend to attenuate x-rays to a lesser degree than bone and iodine contrast agent. Thus, performing two imaging scans, one at a higher tube voltage level (for example, 110-150 kVp), and another at a lower level (for example, 60-80 kVp) will provide more information about the materials being scanned than a conventional CT scan.
Dual energy projection data can be used to reconstruct images using basis material decomposition (BMD) algorithms. The generated images are representative of a pair of selected basis material densities. In addition to material density images, dual energy projection data can be used to produce a new image with X-ray attenuation coefficients equivalent to a chosen monochromatic energy. Such a monochromatic image includes an image where the intensity values of the voxels are assigned as if a CT image were created by collecting projection data from the subject with a monochromatic X-ray beam.
In the medical imaging field, dual energy CT scans are frequently performed at a low energy level of around 80 kVp, and at a high energy level of around 140 kVp. From the images obtained during these scans, it becomes possible to generate basis material density images and monochromatic images (i.e., images that represent the effect of performing a computed tomography scan with an ideal monochromatic tube source). Given a pair of material density images, it is possible to generate other basis material image pairs. For example, from a water and iodine image of the same anatomy, it is possible to generate a different pair of material density images such as calcium and gadolinium. Similarly, from a pair of basis material images, it is possible to generate a pair of monochromatic images, each at a specific energy. The inverse is also possible, i.e. from a pair of monochromatic images, a pair of basis material image pairs can be derived, or a pair of monochromatic images at different energies.
Occasionally, however, it can be helpful to also generate images as if the patient were scanned using another tube spectra without actually having to do the additional scan. For example, in certain instances it might aid a radiologist to view an imaged object at a conventional energy level of 120 kVp. Typically, this would require an additional scan to be performed at the desired energy level. This is a time-consuming step that can further expose a patient to undesired levels of radiation. Further, because time will have elapsed since the initial imaging procedure and circumstances will have changed, it will be impossible to capture the image exactly as it was obtained in the original dual energy CT scan.
As a result, there exists a need for generating an image result as if a CT scan was performed at an arbitrary energy level or spectra, using the imaging results obtained from a dual energy scan.